1. Field of the Invention
The invention concerns a way of conducting split- and splitless sampling (with external vaporization) on capillary columns by syringe with improved precision and accuracy of quantitative analysis.
2. Background Information
For certain, especially, less diluted samples ranging widely in volatilities of the analytically significant components, the application of split sampling is the simplest technique for regulating the sample amount that is to enter capillary columns in order to avoid overloading. (D. H. Desty, A. Goldup, B. A. F. Whymann, J. Inst. Petroleum, 45 (1959 ) 287; I. Halasz, W. Schneider, Anal. Chem., 33 (1961) 987; L. Ettre, W. Averill, ANal, Chem., 33 (1961) 680; G. Schomburg, H. Husmann, H. Behlau, J. Chormatogr., 203 (1981) 179; K. Grob Jr., Proceed, 4th Int. Symp. Capillary Chromatogr. Hindelang 1981, p. 185;). The sampling volume or the sample amount which is actually entering the column can be varied in a double manner, either by the volume which is adjusted within the syringe cylinder or, over a wider range by the splitting ratio, i.e., the ratio between the splitting-flow of the carrier gas leaving the system, and that smaller part of the carrier-gas-flow that enters the column. Splitting ratios are usually varied within the range of 1:10-1:500.
For various instrumental reasons the method of split sampling onto capillary columns had been recognized in the past as insufficient and unreliable for the quantitative analysis of samples widely ranging in component volatilities. The application of this method therefore required elaborate additional calibration measurements in order to eliminate the systematical errors involved in the classical mode of execution of split sampling. The problems arising with split sampling at the quantitative GC-analysis using capillary columns will be discussed in more detail below. The subject of the invention to be described here, is a simple device to remove most of the sources of systematical errors arising at quantitative analysis.
Using the split sampling, mainly such samples are dosed into a GC-system, which are not highly diluted by a matrix solvent or such which must usually be diluted by a solvent in order for the cold "on-column"-technique to be applied for sampling. (G. Schomburg, H. Behlau, R. Dielmann, H. Husmann, F. Weeke, J. Chromatogr., 142 (1977) 87; K. Grob, K. Grob Jr., J. Chromatogr., 151 (1978) 311; K. Grob, HRC CC, 1 (1978) 263).
The procedure of split sampling can be described as follows: The sample volume is adjusted within a common syringe of 1-10 .about.1 total volume; about 0.1-1.0 liters of the sample being dosed. The syringe needle is then introduced via the septum into the hot vaporization chamber of the injector. The sample is extruded from the syringe into the vaporization chamber by piston moving. The sample is vaporized and homogeneously mixed with the carrier gas, e.g., Co.sub.2, and or nitrogen. Thereby droplet (aerosol) formation is prevented by a packing of deactivated glass or quartz wool. The vaporization temperature within the insert (200.degree.-300.degree. C.) must be adjusted according to the volatility of the less volatile sample constituents.
The mixture of carrier gas-sample vapour, is quickly transferred from the vaporization part of the insert into its split region by the high carrier-gas-flow, there, where the splitting takes place, according to the splitting ratio (ratio of splitting flow to column flow of carrier gas). A minor part of the sample thus enters the column, the major part of the sample leaves the system via the split exit.
The column itself (but not the column inlet which protrudes into the heated vaporization chamber) can be maintained at temperatures which are usually lower than the injector temperature is during the sample transfer into the column. The column temperatures may also be low, if temperature programming is to be applied for the separation. The injector temperature must be high, if components of low volatility are, however, contained within the sample.
The criteria for optimized split sampling are:
(a) The absolute sample amount (measured by the total of peak areas) which actually enters the column, must be well defined and reproducible. It must also correspond to the adjusted syringe volume as well as to the splitting ratio. Low standard deviations (5%) of the absolute amounts of the significant sample components, are desirable. In trace analysis such standard deviations may be sufficiently low to avoid the application of the internal standard method for quantitation.
(b) The relative amounts of the sample components which are measured by relative peak areas must not, when on its way from the syringe into the column, be changed by "discrimination" of either volatile or less volatile components. (G. Schomburg, H. Husmann, H. Behlau, J. Chromatogr., 203 (1981) 179). This means that the composition of the original sample may not differ from the composition of that part of the sample that enters the column. The relative peak areas which correspond to the relative concentrations of the constituents must also be reproducible by means of repetitive measurements. The relative standard deviation should be as low as 1% and less for quantitative analyses. Low standard deviations of relative peak areas or amounts of components respectively and even lower systematical errors (i. e., high accuracy) are absolutely necessary to achieve. These rigid requirements were not met by application of the hitherto known devices for split injection onto capillary columns.
(c) The achievement of the optimum separation efficiency of the column may not be prevented by the sampling procedure, especially not with isothermal column operation, because peak focussing (thermal or by solvent effects) is difficult or impossible to attain then.
Sources of errors at quantitative analyses using split sampling:
The syringe needle, which is filled with the liquid sample, must be introduced into the hot injector chamber, which must be kept at even higher temperature, if components of very low volatility are contained in the sample, which otherwise would remain unvaporized within the injector. Temperatures between 200.degree. and 300.degree. C. are usually necessary. From the needle a selective vaporization of the more volatile sample constituents into the carrier gas arises towards the end of the extrusion of the liquid sample into the insert. The less volatile constituents of the sample remain within the syringe needle and are removed from the system during withdrawal of the syringe from the injector. (K. Grob Jr., H. P. Neucom, J. Chromatogr. 195 (1980) 64; G. Schomburg, Proceed. 4th Int. Symp. Capillary Chormatogr., Hindelang 1981 and A. 921). In this way, too many of the volatile components of the sample enter the column. The gas chromatogram obtained in this case appears as though discrimination of the low volatility components has occurred. A deviation from the true composition of the sample is observed. At the same time the standard deviations of the relative as well as the absolute peak areas, as obtained from the chromatograms, are high also. Before the extrusion of the liquid sample occurs, difficulties with selective vaporization of volatile components from the needle may also arise, because the sample is prematurely vaporized by heat transfer from the injector to the needle. Problems, especially, arise with samples which contain low boiling compounds or even dissolved gases. In this case the absolute sample amount that enters the injector, is falsified through formation of gas bubbles. The same can simply happen by thermal expansion of the liquid sample within the needle. It has previously been observed that such sampling errors are higher when hydrogen is the carrier gas, because the heat transfer onto the needle is much faster in this gas for reason of its high thermoconductivity. (G. Schomburg, H. Husmann, H. Behlau, J. Chromatogr., 203 (1981) 179). Further sources of error, which give rise to unreliable quantitative data, are related to the splitting process itself. For example: By the vaporized sample the viscosity of the carrier gas may be differently changed in the splitting or in the column flow. The effective splitting ratio may be different for the earlier and the later vaporized sample components.
Generally, errors at split sampling can be avoided, if the syringe needle is introduced and withdrawn as fast as possible. Moreover, the injector temperature should be as low as possible but considering the volatility of the constituents of lower volatility. (G. Schomburg, H. Husmann, H. Behlau, J. Chromatogr., 203 (1981) 179).
At the 6th Internat. Symp. Capillary Chromatogr., Riva del Garda, an automated split injector was presented by the Hewlett-Packard company, which is capable of very fast syringe manipulation in order to reduce the heat-up of the syringe needle and the related selective vaporization (according to the observations made by Schomburg, which are mentioned above). The provision of temperature decrease, within the insert, can generally not be applied, if the sample contains components of low volatility, which can only be volatilized and transferred into the column at higher temperatures.
The application of matrix solvents of higher boiling point and the avoidance of hydrogen as carrier gas can improve the situation but cannot completely exclude these sources of error.
The cooling of the whole injector, down to temperatures of 80.degree.-100.degree. C. prior to the introduction of the syringe needle and the extrusion of the liquid sample followed by fast heating of the entire injector, including both carrier-gas-flows, revealed, however, that discrimination of the more volatile sample components cannot be avoided, which is probably caused by inhomogeneous heating of the insert (carrier gas of the splitting flow) and of the column inlet (column flow).